Optical finger navigation (OFN) sensors are used to sense relative movement of a user's finger. OFN sensors are mainly implemented using small lenses or optical imaging systems (lenses, prisms, and apertures). The current trend is to implement OFN sensors within devices that have relatively small form factors. However, the use of a small form factor results in design difficulties. For example, the requirements on the physical dimensions of the imaging system as a whole limit the overall size of the OFN sensors.
As the size of OFN devices becomes smaller and smaller, it is harder to design efficient imaging and illumination optics. Also, the integration of these two optical subsystems (e.g., imaging and illumination) becomes more difficult. The direct optical path for the illumination is either not present or would be positioned in a way that results in steep angles inside the transparent cover of the device. Total internal reflection (TIR) inside the navigation cover of the OFN device could limit the amount of light delivered to the navigation surface in such cases.